1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus and an ultrasonic diagnostic method which generate image signals by transmitting and receiving ultrasonic waves to an internal portion of a subject, and more particularly, to an ultrasonic diagnostic apparatus and an ultrasonic diagnostic method which make it possible to reduce an effect of speckle noise generated on occurrence of mutual phase interference between scattering waves produced with a scattering object within the subject.
2. Description of the Related Art
In the ultrasonic diagnostic apparatus, electric pulses are impressed on respective ultrasonic minute oscillating elements in an ultrasonic probe with delay times different from one ultrasonic minute oscillating element to another, and the obtained transmission beam is applied to the inside of a subject. Then, reflected waves from the subject are received by the same ultrasonic minute oscillating element group that has applied the transmission beam, and a reception beam is formed by performing amplification/delay-addition. Furthermore, radio frequency (RF) signal obtained from the reception beam is detected/compressed to acquire an image signal. In particular, the electronic scanning ultrasonic diagnostic apparatus obtains an image inside the subject by electronically scanning the subject with this ultrasonic beam.
FIG. 10 is a block diagram of a conventional ultrasonic diagnostic apparatus.
In a conventional ultrasonic diagnostic apparatus 1, a focus wave surface generator 2 generates a plurality of mutually different pieces of delay time information corresponding to respective members of a ultrasonic minute oscillating element group in an ultrasonic probe 3, and provides the generated information to a pulsar controller 4. The pulsar controller 4 generates a control signal such that a pulsar group 5 generates electric pulses in response to respective pieces of delay time information, and provides the generated control signal to the pulsar group 5.
Then, based on the control signal received from the pulsar controller 4, the pulsar group 5 generates electric pulses, and the generated electric pulses are impressed to the ultrasonic minute oscillating element group in the ultrasonic probe 3, with delay times different from one minute oscillating element to another. As a result, ultrasonic waves are transmitted to a subject (not shown) from the ultrasonic minute oscillating element group in the ultrasonic probe 3, with delay times different from one minute oscillating element to another, and thereby a transmission focus is obtained. Thus, a transmission beam of the ultrasonic waves is formed inside the subject.
Furthermore, reflected waves generated inside the subject are received by the same ultrasonic minute oscillating element group in the ultrasonic probe 3, and are provided to a preamplifier group 6. The preamplifier group 6 amplifies the reflected wave signals received from the ultrasonic minute oscillating element group, and provides the reflected wave signals amplified to a delay circuit group 7. The delay circuit group 7 performs a delay addition of the reflected wave signals, thereby forming a reception beam of the reflected waves. Thus, a scanning line RF signal of the reflection waves is generated in the delay circuit group 7.
Next, the scanning line RF signal generated in the delay circuit group 7 is provided to an image signal detecting unit 8, and the image signal detecting unit 8 detects/compresses the scanning line RF signal to obtain an image signal. The image signal obtained by the image signal detecting unit 8 is given to an image display circuit 9. The image display circuit 9 converts the image signal received from the image signal detecting unit 8 into a luminance signal of an image mapped in accordance with the signal intensities, and gives the luminance signal to a monitor 10. As a consequence, on the monitor 10, an image inside the subject is displayed by luminance in accordance with the signal intensities of the image signal.
On the other hand, a part to be diagnosed inside the subject, e.g., an organ such as the liver parenchyma has structures minute relative to the width of an ultrasonic wave beam to be applied. This is a situation equivalent to one where an infinite number of scattering objects exists in an irradiation region of ultrasonic waves. Once a region having such a minute structures has been irradiated with ultrasonic waves, scattering waves generate from a large number of respective scattering objects, and the scattering waves generated cause a phase interference with one another, thereby incurring so-called “speckle noise”.
This type of speckle noise is similar to speckle noise occurring when laser beams are passed through the atmosphere with fluctuation, and it is attributed to a phase interference among wave surfaces. Typically, the speckle noise comprises speckles each having a size equivalent to that corresponding to the resolution of an ultrasonic diagnostic apparatus, and the average intensity thereof is proportional to the scattering intensity of minute scattering objects. The shapes themselves of the speckles of the speckle noise are not representative of the structure of an organ of the subject. The problem is that the speckle noise impairs the visibility of minute structures inside the subject or the difference in minute scattering intensity.
With such being the situation, in recent years, to reduce the influence of the speckle noise, a so-called “spatial compound” method has been implemented, in which ultrasonic waves are transmitted/received with respect the subject from a plurality of directions, and in which a plurality of images obtained by reflected waves from respective directions are added to one another. (see, for example, Japanese Patent Application (Laid-Open) No. 62-72340 and Japanese Patent Application (Laid-Open) No. 3-99561).
FIG. 11 is a diagram showing an example of wave surface of ultrasonic wave which is to be transmitted to a subject in a perpendicular direction in the case that an image is generated with the conventional spatial compound technology. FIG. 12 is a diagram showing an example of wave surface of ultrasonic wave which is to be transmitted to a subject in the direction of oblique in the case that an image is generated with the conventional spatial compound technology.
Here, for the sake of simplification, suppose that, when an ultrasonic wave is perpendicularly transmitted to the subject, four scattering objects having the same size exist evenly spaced in the different depths Z, and at random position in the direction X perpendicular to the depth direction of the ultrasonic wave.
As shown in FIG. 11, usually, an ultrasonic wave is transmitted so that the wave surface of a plane wave becomes perpendicular to the transmitting/receiving surface direction X of the ultrasonic probe, and scattering waves are caused by the scattering objects existing inside the subject. Moreover, as shown in FIG. 12, to reduce the influence of speckle noise, another ultrasonic wave which is a plane wave is transmitted from an oblique direction Z′ so that it's wave surface tilts with respect to the transmitting/receiving surface direction X of the ultrasonic probe, and scattering waves are caused by the scattering objects existing inside the subject.
In this manner, when ultrasonic waves are transmitted from mutually different directions, scattering waves in response to directions of transmitted ultrasonic waves occur.
FIG. 13 is a diagram showing waveform of the scattering wave produced with the scattering object when the ultrasonic wave is transmitted so that the wave surface of the plane wave becomes vertical to the direction X of the transmission-and-reception face of the ultrasonic probe as shown in FIG. 11. FIG. 14 is a diagram showing waveform of the scattering wave produced with the scattering object when the ultrasonic wave is transmitted so that the wave surface of the plane wave inclines to the direction X of the transmission-and-reception face of the ultrasonic probe as shown in FIG. 12.
As shown in FIG. 13, when an ultrasonic wave is perpendicularly transmitted to the subject, since the four scattering objects exist evenly spaced in the mutually different depths Z, four scattering waves each having a similar waveform occur at regular intervals in correspondence with the ultrasonic wave penetration depths. On the other hand, as shown in FIG. 14, transmitting an ultrasonic wave to the subject in the oblique direction Z′ results in that the four scattering objects exist unevenly spaced at the positions where penetration depths Z′ of the ultrasonic wave are different each other, since the four scattering objects exist at random in the transmitting/receiving surface direction X of the ultrasonic probe. Consequently, four scattering waves having a similar waveform occur at irregular intervals in correspondence with the ultrasonic wave penetration depths Z′.
As a result, obtained are scattering waves (speckle noise) having interference patterns mutually different in accordance with the directions of the transmitted ultrasonic waves.
FIG. 15 is a diagram showing the result of interference on the scattering waves shown in FIG. 13. FIG. 16 is a diagram showing the result of interference on the scattering waves shown in FIG. 14.
As shown in FIG. 15, when an ultrasonic waves is perpendicularly transmitted to the subject, scattering waves that are evenly spaced occur, and they interference with one another, resulting in a regular waveform. On the other hand, as shown in FIG. 16, when an ultrasonic wave is obliquely transmitted to the subject, scattering waves that are unevenly spaced occur, and they interference with one another, resulting in an irregular waveform.
That is, even if the positioning of scattering objects is the same, provided that directions of the wave surfaces of transmitted ultrasonic waves are different, the combination of scattering objects causing scattering changes, and thereby speckle noise based on different phase interferences is obtained. Then, images obtained by thus changing the direction of ultrasonic wave beams are added, so that the reduction in speckle noise, namely, the stabilization of fluctuation is performed statistically by averaging independent images.
FIG. 17 is a diagram showing an example of scanning direction by the ultrasonic wave in case that an image is generated with the conventional spatial compound technology. FIG. 18 is a conceptual diagram explaining the way of generating images with spatial compounding using data obtained by scanning in the scanning direction shown in FIG. 17.
As shown in FIG. 17, for example, when scanning is performed in three different directions, three scattering waves with mutually different interference patterns are obtained. Then, as shown in FIG. 18, three scattering waves obtained from the identical position are added. This allows the reduction in speckle noise.
However, the generation of an ultrasonic diagnostic image by the conventional spatial compound technique involves the following problems.
First, the spatial compound is a technique for obtaining a single image by adding a plurality of images obtained by transmitting/receiving ultrasonic waves with respect to a plurality of directions. However, in order to obtain a speckle reducing effect, the independence of a speckle pattern in an image to undergo addition is required. For this purpose, it is necessary that the angles formed between directions of transmissions/receptions of ultrasonic waves performed over a plurality of times be large to an extent such as to secure the independence of speckle pattern. However, since the aperture of the ultrasonic probe is limited, the number of independent images obtainable is restricted. This causes a problem in that a sufficient speckle reducing effect may be unattainable.
Secondly, with an ultrasonic wave transmitted/received, when the transmission/reception direction is tilted with respect to the transmitting/receiving surface of the ultrasonic probe, it is necessary to perform scanning at an angle larger than the angle of view at which display is practically performed, for image addition. This raises a problem in that the number of frames that can be imaged per acquired data or per unit time decreases. Specifically, when a region filled with dotted lines in FIG. 17 is a display region necessary to be displayed as an image, if the transmission/reception direction of ultrasonic waves is perpendicular to the transmitting/receiving surface of the ultrasonic probe, it suffices to scan the display region alone. However, when the transmission/reception direction of ultrasonic waves is tilted with respect to the transmitting/receiving surface of the ultrasonic probe, it is necessary to scan a region outside the display region.
Conversely, when the transmission/reception direction of ultrasonic waves is tilted with respect to transmitting/receiving surface of the ultrasonic probe, if the display region alone is scanned, there occur portions subjected to no scanning so that the speckle reducing effect varies from one spot of the image to another, resulting in unevenness of the image. Specifically, as shown in FIG. 17, when the display region alone is scanned from three directions, i.e., from the front and right-and-left oblique directions, a speckle reducing effect is obtained with respect to a region where the three scanning lines are intersected, whereas with respect to a region where these scanning lines are not intersected, no speckle reducing effect can be obtained.
A third problem is that, in order to obtain the image of a single point, transmitting/receiving operations of an ultrasonic wave over a plurality of times are required, thereby deteriorating real time characteristic. As shown in FIG. 17, when scanning the display region alone from three directions, i.e., the front and right-and-left oblique directions, three transmitting/receiving operations are needed to obtain the image of a single point.